Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
As with any vehicular transmission, it is desirable in a hybrid transmission to measure rotational speed of the output shaft or a member that is ratiometrically synchronized therewith in its rotation in order to determine vehicle speed and provide needed information regarding the transmission operation for use in its control. Various technologies are known for providing such speed information including variable reluctance (VR) sensors, magneto resistive (MR) sensors, and hall effect (HE) sensors. In all such sensors a target wheel comprising alternating regions of high and low permeability (e.g. toothed wheel) rotates in proximity to a sensing element to generate a pulse train in accordance with the target wheel rotation. For strict speed sensing where position is not a desired metric to be measured, the target wheel is generally uniform in the distribution of the high and low permeability regions. Other distribution patterns are generally reserved for encoded applications which can discern position or angular rotational information therefrom.
With respect to a transmission output, and perhaps other transmission members, accurate speed detection is desired and, while angular position is not, direction of rotation is a desired metric for measurement. As such, it is common practice to employ a pair of such sensors separated by a predetermined electrical angle which allows for determining the speed and direction of rotation—the speed being essentially a frequency based signal and the direction being a relative event based signal.
Full range speed sensing may be critical in certain applications such as output speed sensing in a transmission. With respect to hybrid transmissions, this is true since accurate speed control—itself a critical factor in hybrid transmission operation—requires precise measurements down to and through zero vehicle speed. In this regard, MR and HE sensors are truly zero-velocity sensors since the output signal amplitude is substantially consistent and detectible regardless of the target wheel speed whereas (VR) sensors have an output whose amplitude decreases with decreasing speed and eventually is undetectable at lower speeds. Additionally, HE and MR sensors are generally well adapted to diagnosis through direct measurement means without interfering with the speed measurements whereas VR sensors do not lend themselves as readily to easy monitoring and automated fault detection. However, HE sensors generally rely upon an actively magnetic target wheel and are not commonly employed in automotive speed sensing applications. MR sensors generally require some controlled current or voltage source and are sensitive to temperature and air gap fluctuations. VR sensors generally do not suffer from the same shortcomings of HE and MR sensors. Additionally, VR sensors are generally more robust in an automotive environment of vibration and high temperatures.